Hitherto, a buildup multi-layer printed circuit board has been manufactured by a method disclosed in Japanese Patent Laid-Open No. 9-130050.
That is, an interlayer resin insulating layer is built on a core substrate having through holes formed therein. Then, a circuit pattern is formed on the interlayer resin insulating layer. The foregoing process is repeated so that the buildup multilayer printed circuit board is obtained.
At present, the through holes are formed in the core substrate by forming penetrating openings by using a drill. Therefore, the smallest limit of the diameter of the penetrating opening is 300 μm. The density of the through holes cannot be raised to a value larger than a value which is defined by the diameter of the drill. Hence it follows that a method using a laser beam to form the core substrate has been investigated. Since the core substrate has a thickness of about 1 mm, fine through holes cannot, however, easily be formed.
On the other hand, the multi-layer printed circuit board, which is employed as the package substrate, must efficiently diverge heat generated in the IC chip. The multi-layer printed circuit board incorporates a core substrate which is constituted by a laminated resin board having a thickness of about 1 mm and on which an interlayer resin insulating layer having a thickness of several tens of μm and a circuit layer are laminated. Therefore, the thickness of the multi-layer printed circuit board is mainly made up by the core substrate. That is, the core substrate causes the thickness of the multi-layer printed circuit board to be enlarged and the thermal conductivity to be decreased.
To solve the above-mentioned problems, an object of the present invention is to provide a multi-layer printed circuit board and a method of manufacturing a multi-layer printed circuit board capable of raising the density at which the through holes are formed and reducing the thickness thereof.
To prevent the occurrence of the warpage and the crack of the upper interlayer resin insulating layer and conductor circuits.
When through holes are formed by laser beams, disconnection sometimes occurs in a heat cycle or the like. Thus, satisfactory reliability cannot be obtained. The cause of the disconnection has been investigated, resulting in detection of mixture of air bubbles in the resin with which the through hole is filled.
The cause of mixture of the air bubbles has furthermore been investigated by the inventor of the present invention. As a result, the mixture is caused by residual burrs of copper foil inwards extending from the opened portion of the through hole when the through holes are formed in a copper-clad laminated board which constitutes the core substrate. That is, as shown in FIG. 70 (A), when a through hole 633 is, by a laser beam, formed in a core substrate 630 constituted by laminating copper foil 632, a burr 632b of the copper foil 632 undesirably left at the edge of the opening of the through hole 633. A tapered through hole 633 is sometimes formed. When a plated film 635 is formed to form the through hole 636 as shown in FIG. 70 (B), air bubble E is sometimes left between the burr 632b and the plated film 635. When the through hole 636 is filled with a resin filler 640 as shown in FIG. 70 (C), air bubble E is sometimes left between the reverse side of the portion including the burr 632b and the resin filler 640. As shown in 70 (D), the portion including the burr 632b of the copper foil extending inwards sometimes prevents smooth injection. In the foregoing case, a portion which is not filled with the resin filler 640l is detected in the through hole 636. Thus, a fact has been detected that the reliability in the connection of the printed circuit board deteriorates owing to the air bubble and the non-filled portion.
Since the hole forming operation using the laser beam is performed, an oxidation-reduction layer is formed as a process which is performed before the laser beam is applied and the number of laser shots is increased. Thus, the process takes a long time and the cost is enlarged.
To overcome the above-mentioned problems, the inventor of the present invention has attempted to employ a BT (Bismaleimide-Triazine) resin board as the core substrate. A fact has, however, been detected that the BT resin board having a flat surface encounters deterioration in the adhesiveness with a metal film formed on the right side of the core substrate. Therefore, the inventors or the present invention have attempted to improve the adhesiveness with the metal film formed on the right side of the core substrate by employing a resin film constituted by dispersing soluble particles in refractory resin. Thus, there arises problems in that the resin film, however, encounters decrease in the strength required for the core substrate and the interlayer resin insulating layer cannot be formed in the upper layer.
To overcome the foregoing problems, another object of the present invention is to provide a printed circuit board and a method of manufacturing a printed circuit board capable of forming adequately forming through holes by using laser beams.
Since the core substrate is filled with core materials, such as glass cloths, the laser beam must be applied to each hole for a long time to form the through holes in the core substrate having a thickness of about 1 mm by using laser beams. To form hundreds of through holes, an excessively long machining time is required and the manufacturing cost cannot be reduced. On the other hand, the through holes each having a small diameter sometimes encounters disconnection during the heat cycle. Therefore, satisfactory reliability cannot be realized as compared with conventional through holes formed by drilling and each having a large diameter.
To achieve the foregoing objects, a still further object of the present invention is to provide a multi-layer printed circuit board and a method of manufacturing a multi-layer printed circuit board capable of improving high-frequency characteristics of a ground line and a power supply line and preventing a malfunction of an IC chip caused from insufficient quantity of electric power to be supplied.
On the other hand, the diameter of each penetrating opening can be reduced as compared with an opening which is formed by drilling when the penetrating openings are formed in the core substrate by using laser beams. When the penetrating openings are formed in a copper-clad laminated board employed to serve as the core substrate, the number of laser shots are increased excessively and an excessively long time is required. When the openings are formed by the laser beams, a portion of copper for forming copper foil on the surface of the inner wall of the through hole is undesirably left. It leads to a fact that undesirable separation of the plated film formed in the through hole takes place.
To overcome the foregoing problems, its still further object of the present invention is to provide a printed circuit board excellent in connection characteristics and reliability and permitting a high-density structure which is manufactured by forming openings in a resin plate which serves as a core substrate by using laser beams and by performing sputtering to for a sputtered layer to form penetrating opening each having a small diameter and which is free from any separation of the plated films in the through holes and a method of manufacturing a printed circuit board.